Reversible electric power converters are usually connected between a DC positive terminal, a DC negative terminal, and a plurality of AC terminals, for example three AC terminals. The voltage at each AC terminal is controlled through a respective phase portion of the converter, which is structurally identical to and independent from the other phase portions. For simplicity, the known converters and the converters of the invention will be described with reference to one AC terminal.
The converters provide an output voltage at the AC terminal which is not sinusoidal, since it varies according to a limited number of voltage levels. This generates high frequency harmonic components which are detrimental to the quality of the AC voltage, and which usually requires appropriate harmonic filters.
The voltage levels which are outputted at the AC terminal are in general those of the DC positive and negative terminals, and optionally a number of intermediate voltages, for example the average voltage between the DC positive and negative terminals.
The high frequency components can be reduced by increasing the number of voltage levels outputted by the converter. In this way the quality of the AC voltage is improved, and the cost of the filters may be reduced.
The paper “Loss balancing in three-level voltage source inverters applying active NPC switches” (T. Bruckner, S. Bernet, 17-21 Jun. 2001) discloses an active neutral point clamped converter (ANPC converter). In the ANPC converters, positive and negative capacitors are connected in series between the terminals of a DC source, and they provide a neutral point therebetween.
A positive cell and a negative cell are connected to the positive and negative capacitors, and a selector is connected downstream the positive and the negative cells. An ANPC converter outputs three voltage levels, i.e. the voltages of the DC positive and negative terminals and of the neutral point.
U.S. Pat. No. 7,292,460 discloses an improvement to the ANPC converters, where the positive cell, the negative cell and the selector may comprise many capacitors and couples of switch units. The switch units comprise each a controllable switch with an antiparallel diode. With an appropriate control, the voltage of the capacitors may be added to, or subtracted from, the typical output voltages of an ANPC converter. The number of output voltage levels is thereby increased.
U.S. Pat. No. 6,958,924 discloses a stacked multicell converter (SMC converter), where several stages of switching cells are connected between a DC source and an AC source. In each cell two switch units are provided, which are controlled to be always in opposite states.
The stages define two end groups of switch units and at least one intermediate group of switch units which are common to adjacent stages. Capacitors are connected in parallel to the switching cells of the stages, and they have voltages increasing from the AC source to the DC source. All the stages are short-circuited at the AC terminal.
U.S. Pat. No. 7,313,008 discloses a converter having two end switching groups composed by diodes and an intermediate switching group comprising two controllable switches. Capacitors connect the switching groups at the DC side, while at least the end switching groups are short-circuited at the AC terminal.